A. Field of the Invention
The present invention relates to a CIS flatbed scanner, especially to a CIS scanner having rolling elements interposed between a CIS module and a sheet table, thereby to reduce friction therebetween while rolling over the bottom surface side of the sheet table.
B. Description of the Prior Art
It is known that the most commonly used photoelectric converting devices for flatbed scanners are CCD line image sensor and CIS contact sensor. A flatbed scanner preferably uses a CCD line image sensor because its scene depth is deeper and can generate better picture qualities of images. On the other hand, a CIS contact sensor is mostly used in fax machines or in paper-feeding type scanners because its scene depth is much shorter and its resolution louver.
Nevertheless, the trade-off for using a CCD line sensor is that the size of the flatbed scanner is restricted by the light path required for a CCD line sensor. For this reason, the size of the flatbed scanner can hardly be reduced in a great scale even with the aid of mirror arrays to reflect the light beams.
Recently, a new approach is trying to replace a CCD line sensor with a CIS contact sensor for producing flatbed scanners without the requirement of high image qualities. An advantage for this new approach is that it can save a lot of time and efforts in assembly and maintenance because a CIS contact sensor is modularized. All the necessary optical elements, such as mirrors, light source and a photoelectric converting device, are included in a compact module. Thus, a CIS module can help to simplify the inner structure of a scanner. Moreover, a CIS contact sensor is smaller and lighter than a CCD line sensor. With a CIS contact sensor, the scanner can be portable with height around 30 mm. Nevertheless, this new approach did not prevail. The main reason is because the scene depth of a CIS contact sensor is only about 0.3 mm. Based on the structure of a conventional flatbed scanner, it is very difficult to assure the sheet remains within the scene depth of a CIS module. Even a small manufacture error or structure distortion may affect the picture quality of the images.